1. Field of the Invention
The present invention relates generally to a controller for fluorescent tubes and, more particularly, to a fluorescent lamp dimming system for use with a standard "rapid start" ballast system. The system of the invention uses a switching circuit to modulate the main AC line input in a manner which preserves cathode heater voltage during dimming and maintains reasonably clear sinusoidal line current without affecting the power factor.
2. Description of the Prior Art
In the prior art most controllers of inductive fluorescent lighting loads have energized these loads at a variable phase angle turn on point and allowed natural commutation, i.e. the zero crossover point in the current and/or voltage cycle, to switch off the load. Other controllers have turned the fluorescent lighting loads on at the zero crossover point and have turned the loads off at a selected phase angle to yield the desired amount of lighting control.
Phase angle turn on controllers, i.e. controllers of the first type described above, which power rapid start ballast fluorescent lighting systems have a very limited control range because the regulatory effect of the choke-capacitor components in the ballast tends to counteract any change in RMS load voltage and because there is a rapid drop in fluorescent tube heater voltages as the angle of turn on is increased.
Phase angle turn off controllers, i.e. controllers of the second type, produce a stronger counter electromotive force spike from the rapid start ballast inductance. This spike causes severe acoustic noise and break down of circuit components. Prior art devices have heretofore suppressed this spike with some loss of power. However, rapid start ballast circuits connected to a turn off type of controller employing spike suppression experience low fluorescent tube heater voltages at lower dimming levels resulting in a reduced dimming range.
Thus, in the prior art generally it has not been possible to achieve effective and reliable dimming control of rapid start fluorescent ballast lighting systems because the reduced operating voltages imposed upon the fluorescent tube electrodes at lower light levels causes poor tube ignition, causes premature tube drop out, and lessens tube life due to cathode stripping. The regulator effect offered by the series connected choke and capacitor arrangement in the rapid start ballast opposes attempts to control or modulate the ballast AC input. Extremely high amplitude counter electromotive forces or flyback spikes resulting from the turn off control of rapid start ballast systems causes unacceptable ballast acoustic noise levels, causes poor fluorescent tube crest factors, and endangers circuit components. A further disadvantage of existing control methods is that low light levels are susceptible to light intensity changes caused by line voltage changes.
One invention contained in U.S. patent application Ser. No. 548,523, filed Nov. 3, 1983 and assigned to the same assignee as the present invention overcomes some of the difficulties of the prior art. That invention relates to a system wherein the counter electromotive switching spikes generated as a result of phase turn off of a reactive type load such as that found with respect to fluorescent tubes are converted into power pulses which are utilized during switch off intervals to achieve corrective work functions such as maintaining the filaments of a fluorescent tube heated during the switch off intervals.
That system, however, while it does increase the turndown range for dimming while maintaining required cathode heating voltage, it does not reach the desired combination of maintaining cathode heater voltage during full range dimming, i.e. from 100% to about 10% output, together with the provision of a reasonably clean sinusoidal line current having a good power factor.